WO2010058131A1

WO2010058131A1 - Method for reconfigurable random texturing of a photochromatic component and security and/or authentication method implementing same

Info

Publication number: WO2010058131A1
Application number: PCT/FR2009/052222
Authority: WO
Inventors: Thierry Fournel; Yann Boutant; Laurence Bois; Nathalie Destouches-Castagna
Original assignee: Universite Jean Monnet; Centre National De La Recherche Scientifique; Universite Claude Bernard Lyon I; Signoptic Technologies
Priority date: 2008-11-20
Filing date: 2009-11-19
Publication date: 2010-05-27
Also published as: FR2938679B1; EP2359317A1; EP2359317B1; FR2938679A1

Abstract

The present invention relates to a method for texturing a hardware component comprising a photochromatic material (2) including a stable array (2a) containing chaotically distributed photosensitive agents (2b), characterised in that the photochromatic material is isolated by means of a variable light field such as to migrate and reduce the photosensitive agents within the array to achieve a novel random arrangement of said photosensitive agents in said array. The invention also relates to a method for reconfiguring a random texture of a photochromatic hardware component, as well as to a security and/or authentication method implementing such random texturing methods.

Description

Reconfigurable random texturing method of a photochromic element and security and / or authentication method implementing this method

The present invention relates to a method of random texturing a photochromic material element, and a method of securing and / or authentication of data or objects using such a method of generating random textures.

The object of the invention finds especially particularly advantageous applications in the fields of securing objects or data and / or their authentication.

The method of the present invention thus allows the generation of random macroscopic states by modifying at least a part of the structural characteristics of a photochromic subject material element by means of a light field with random spatial intensity modulation, or to identify this subject matter element, either to jointly identify the subject material element and a secondary hardware element used to shape the incident incident inhomogeneous light field. The structure of the material element thus modified retains variability and stability at the obervation scale so that acquisition of a signal at this scale provides a random texture image characteristic of the subject material element.

The signal obtained can be used in particular to identify the subject material element itself, where appropriate in the context of the authentication of manufactured products or for the securing of information by means of the acquisition signal obtained, characteristic of the modified structure of the subject material element.

This problem of "identification and / or securing by the material" requires that the subject material element present at the scale of observation, and typically in a range of scales ranging from micrometer to millimeter, discriminating structural characteristics defining the macroscopic state of the material element subject, in terms of morphology, chemical or physico-chemical composition, or color induced by morphology and / or chemical or physicochemical composition.

In the field of authentication of manufactured products, it must be impossible in practice or almost impossible to physically reproduce these characteristics.

In the state of the art, it has already been proposed various solutions to secure and / or authenticate by a random signature extracted from the material objects or data.

Patent applications FR 2895543 A1 and FR 2866139 A1 disclose processes for extracting random signatures and for protecting against the reading of sensitive information from natural materials, especially fibrous materials such as paper, which exhibit a great structural complexity, a chaotic aspect at different scales, and above all a quasi-invariance. However, these methods are limited by their irreversible nature. Indeed, it is a question of protecting oneself from the threat of a potential lure of reading following the diversion of an acquisition of the structure of the material element subject on a macroscopic scale during a previous reading.

Other identification and authentication technologies are also known based on the intrinsic characteristics of the material. However, these technologies are also irreversible.

Also known are techniques for recording and recording data by illuminating photosensitive materials using light beams. Examples which may be mentioned are the processes described in documents US 2004/043301; US 7,052,812 and

WO 03/058292.

These different processes all rely on principles of illumination and texturing of deterministic materials. In particular, in these various methods, the light fields used to write the data in a photosensitive material are structured according to the data to be recorded, that is to say that the properties of the light fields are determined by the data to be recorded before illumination. photosensitive materials recording. The texturing of these materials is therefore directly related to the nature of the data entered and can be reproduced.

The present invention aims to provide a solution of identification by the material which does not suffer the disadvantages encountered with the techniques of the prior art. The invention aims in particular to provide an identification solution by a unique code or identifier, of a random nature and substantially impossible to reproduce identically.

Thus, according to a first object, the invention proposes a process for the random texturing of a photochromic material element comprising a stable matrix in which moving photosensitive agents are chaoticly distributed. According to this method, the photochromic material is insulated by means of a variable and random light field causing the reduction of the photosensitive agents within the matrix and their random migration to reach a new random arrangement of said photosensitive agents in said matrix.

Here we mean by random light field that its intensity spatially varies randomly. Thus, according to the method of the invention, the texture imparted to the photochromic material of the subject material element may depend on two independent random parameters that are on the one hand the insolation light field and on the other hand the random arrangement photosensitive agents within the matrix of the photochromic material. It is thus possible to extract from the random structure obtained a unique, irreproducible identification code or signature, insofar as both the light field and the migration of the light agents are random and independent. In addition, once a first texturing given, it is possible to replace it with a new simply by proceeding to a new illumination using the random light field. The resulting texture is not fixed but reversible and reconfigurable in a random manner. In a preferred embodiment of the method of the invention, the light field of insolation of the photochromic material element results from the light transmitted and / or reflected by an intermediate material element. placed on the optical path of an incident light beam and whose physical characteristics spatially vary the intensity of the light field.

As a variant, the variable and random light field may also result from the light transmitted and / or reflected by an optical and / or digital system using a hardware or digital key.

Advantageously, the light beam is preferably a coherent or quasi-coherent light beam, the resulting light field of insolation then being a field of scab. More preferably, the incident light beam is a structured incoherent light beam.

According to a particular characteristic of the method of the invention, the intensity of the spatially and / or the divergence of the insolation light field is varied by means of an optical system. Still according to the method of the invention, the exposure of the photochromic material element is advantageously effected by means of a light field whose spectrum is at least partially located in the ultraviolet range.

It is also advantageous to stabilize, where appropriate, the random arrangement of the photosensitive elements in the photochromic material element after insolation, on the one hand, by a chemical and / or physical treatment intended to eliminate the non-reduced photosensitive agents and / or on the other hand by a physical and / or chemical treatment intended to protect the reduced photosensitive agents from parasitic light radiation. According to another preferred feature of the invention, the photochromic material element comprises a titanium oxide matrix (TICH) porous the pore size is less than 10 ^2nm and containing photosensitive agents such as salts of silver, gold or platinum, or even silver complexes. A second subject of the present invention also relates to a method for generating a new random arrangement of reduced photosensitive agents in the stable and porous matrix of an element. photochromic material whose structure has been modified according to the random texturing method described above. This method is characterized in that the random arrangement of reduced photosensitive agents obtained after a first insolation of the photochromic material element is erased and in that a further insolation is carried out using a spatially variable light field.

According to a preferred form of implementation of this method, the prior random arrangement of the reduced photosensitive agents by exposure of the photochromic material element by means of a light beam whose spectrum is located at least partially in the visible range is erased. .

Still according to this method and in particular cases of implementation thereof, it is eliminated, if necessary before erasing, the protection of the reduced photosensitive agents introduced beforehand during an earlier stabilization step as described above and / or the matrix of the photochromic material element is reloaded into non-reduced photosensitive agents, in particular following an elimination of such agents during a said stabilization step.

A third subject of the invention also relates to a method for securing and / or authenticating data (s) or object (s) in which: a) a figure of a variable light field is written spatially within a photochromic material element according to the previously defined random texturing method, said figure being deformed according to the arrangement of the photosensitive agents and imparting a random texture to the photochromic material element; b) the photochromic material element is probed; c) extracting a signal or an image of the random texture and / or a signature extracted from this texture to secure an object and / or data. According to a preferred characteristic of this method, in step c) the signal or the image of the random texture and / or the extracted signature is recorded as a reference in a database and / or on the element photochromic material or on an object to be secured and / or authenticated, then:

the photochromic material element is again probed;

a signal or an image of the random texture and / or an extracted signature is formed;

the signal or the image of the random texture and / or the extracted signature is compared with the signal or the image of the random texture and / or with the extracted signature recorded as a reference to authenticate the object.

According to a preferred feature of this method, the recording step is done according to a secure protocol.

According to another preferred feature of this method, the random texturing of the photochromic material element according to the previously introduced technique is erased and steps a) to c) are again carried out.

Still according to the security method of the invention, a random signature is extracted from the random arrangement of the reduced photosensitive agents in the photochromic material element, the random signature constituting a key or a cryptographic function parameter (s) and / or steganographic (s).

Still according to the invention, the photochromic material in which is registered a random texturing is deposited on at least part of an object to secure and / or authenticate.

The implementation and exploitation of the methods of the invention find a particularly interesting application in the context of the creation and operation of graphic codes such as bar codes or Datamatrix codes for example. Indeed, according to the invention and the previously defined methods thereof, it is possible to produce a graphic code of any type, composed of a unitary unit arrangement determined and which, according to the invention at least one unit element is textured in accordance with the random texturing method of the invention and may be, if necessary, modified and reconfigured in accordance with the method of generating a new random arrangement of the invention. The production of such a graphic code, of which at least one element is randomly textured in accordance with the invention, makes this code virtually tamper-proof. Indeed, it is possible by playing on the variability of the insolation light field to give a third dimension of texture to the unitary elements of the code. Such a texture is impossible to reproduce without knowing the illumination parameters used to generate this texture or the random arrangement of photosensitive agents in the photochromic material at the level of the unitary texture element of the code.

Various other features appear from the description given below with reference to the accompanying drawings which show, by way of non-limiting examples, embodiments of the object of the invention.

FIG. 1 is a schematic representation of the method for writing a random texture in a photochromic material of the invention,

FIG. 2 represents a scab pattern recorded on a matrix sensor located in the scab field;

FIG. 3 represents an image of the random macroscopic state of a material element subject photochromic after inscription of a random texture according to the method of the invention,

FIG. 4 diagrammatically represents a section of the photochromic material layer that is useful for carrying out the process of the invention and seen in the microscopic state,

FIG. 5 is a view similar to that of FIG. 4 showing the microscopic state of a layer of photochromic material, after implementation of the method of the invention. FIG. 6 represents an element insolation device subject photochromic for implementing the random texturing method of the present invention,

FIG. 7 is a photograph of a random macroscopic state of a photochromic material obtained by exposure to UV light in accordance with the random texturing method of the invention,

FIGS. 8A and 8B show two scab figures inscribed in a subject photochromic material according to the process of the invention and having different characteristic dimensions observed at the same scale,

FIGS. 9A and 9B respectively show an image of a portion of a photochromic material on a subject material element taken before and after erasing a random texture by insolation at 488 nm in visible light.

According to a first object, the present invention relates to a method of modifying the structure of a subject material element exhibiting properties or being coated with a material with photochromic properties. This method essentially consists in freezing in a said material element photochromic subject by light insolation a signal or a noise which will be deformed by the material of said material element so that the state of the structure of the material element is after exposure, at the same time characteristic of the fixed noise and the material element itself. From this modified material element, it is possible to make a photograph of the texture of the modified material element and by means of this photograph to extract a signature allowing the securing and / or authentication of an object on which the material element is affixed or data for example. As shown diagrammatically in FIGS. _1, 4, 5 and 6, the method of the invention consists in insoling a subject material element 1 constituted or coated with a photochromic material 2 by means of a highly variable light field 3. The material element subject 1 can in particular be in the form of a massive sample to identify or secure for example, or in the form of a deposit of photochromic material 2 secured to a substrate 4.

As is apparent schematically from FIGS. 4, 5 and 6, the photochromic material 2 preferably has, at least in range of scales smaller than the observation scale, typically the nanometer, the ten nanometers or even the hundred or so nanometers, two components, one rigid or quasi-rigid and porous which forms a matrix 2a, the other non-rigid, consisting of a set of photosensitive agents 2b, mobile under certain conditions within the matrix 2a under the effect of an insolation light field 3. The matrix 2a and the photosensitive agents 2b thus determine the microscopic state of the photochromic material 2 as well as the macroscopic state of the film. material element subject 1, and in particular its morphology. Here, the term "observation scale" refers to the field of far-field optical imaging.

During an exposure of a said subject material element 1 comprising a layer of photochromic material 2 by means of a variable light field 3, the said light field causes, within the matrix 2b of the photochromic material, a migration and a reduction of the agents photosensitive 2b to achieve a new random arrangement of said photosensitive agents 2b in said matrix 2a, thus conferring on the photochromic material 2 and the subject element 1 a new structure, observable at the microscopic and macroscopic scales respectively. Generally, any insolation causes the change of atomic state of all or part of the agents 2b as well as their migration within the matrix 2a of the photochromic material. The distribution of the agents 2b in the matrix 2a is random, so that the microscopic and macroscopic states of the photochromic material 2 and the material element subject 1 themselves are random, both before insolation and after insolation. Among the matrices 2a that can be used to produce the photochromic material 2 that is useful for the process of the invention, mention may be made, for their photocatalytic characteristics, of various metal oxides, in particular TiO ₂ titanium oxide. As photosensitive agents 2b, it is preferable to use the silver salts (nitrate, acetate, perchlorate, chloride, etc.) and the silver complexes, but also the gold or platinum salts. . In addition, the use of co-catalysts such as gold, palladium and platinum may make it possible to improve the efficiency of the photochromic phenomenon within the photochromic material.

The luminous intensity in a section of the illuminating light field 3 of the subject material element 1 constitutes within the scope of the invention a "figure" highly variable, whose resolution is modifiable by moving the material element subject 1 in the light field 3.

If the "figure" defined by the intensity of the illuminating light field 3 may result essentially from the nature of an incident light 6 illuminating the material element subject I ₁ this figure may also be generated and modified, using or not an optical system, by illumination of an "intermediate" or "secondary" material element 5 arranged on the optical path of an incident light beam 6 between the source of said beam 6 and the material element subject 1. In this second case, the intermediate material element 5 constitutes by its physical characteristics (roughness, heterogeneity, etc.) a source of strong variability of the light field 3 resulting from the interaction between the incident light beam 6 and said material element 5.

When the intermediate material element 5 is a diffusing element illuminated in coherent or substantially coherent or partially coherent light, the resulting light field 3 is then a so-called "speckle" or "scab" field in French. It is then these scabs that constitute a writable figure in the photochromic material 2 of the subject material element 1. Such a scabbard figure, recorded on a matrix sensor placed in a scabbed field, is for example shown in FIG.

The light field 3 may also result from the scattered light after transmission or reflection of a non-coherent incident light 6 on the intermediate material element 5. An optical system not shown in FIGS. 1 and 6 may also be positioned after the intermediate material element 5 for adjusting the divergence or increasing the variability of the insolation light field 3.

As is thus apparent from the foregoing, the primary purpose of the insolation light field 3 is, as part of the method of the invention, to randomly modulate the concentration per unit area of the photosensitive agents 2b within the matrix 2a of the material. photochromic 2 of the material element subject 1 subject to insolation. This modulation causes a migration phenomenon and a chemical reduction of the photosensitive agents 2b within the porous matrix 2a, causing, as is more particularly apparent from FIGS. 4 and 5, the formation of particles 7 whose concentration and distribution in the matrix 2a depend both on the intensity modulation of the insolation light beam 3, therefore on the incident figure inscribed in the photochromic material 2, and on the spatial distribution of the photosensitive agents 2b in said matrix 2a at the time of the insolation. therefore, the current microscopic state of the photochromic material 2 of the subject element 1. The method of the invention thus leads to freeze in the porous matrix

2a of the photochromic material 2 a random incident light figure which is further deformed during its registration in the material 2 according to the microscopic state of the latter.

At the macroscopic observation scale, the combined phenomena of reduction and migration of the photosensitive agents 2b within the matrix 2a of the photochromic material 2 under the insolation of a light field 3 confer a stable "random macroscopic state" of photochromic material 2, observable in three dimensions or less, of which it is possible to make an image conforming to that of FIG. 3, exploitable for the purpose of securing and / or authenticating the subject material element supporting or comprising the material photochromic or recorded data on any type of media.

The macroscopic state of the photochromic material on the subject material element is qualified as random, in the sense that it is unpredictable a priori, that due to the randomness of the initial microscopic state of the photochromic material 2 and on the other hand the high variability of the illuminating light field 3 implemented.

The photo-inscription of a luminous figure may be limited on the subject material element 1 to a reduced zone of the photochromic material 2 or a set of reduced connected areas of any shape on the surface of said material 2. The photo-inscription on the subject material element 1 of a figure limited to a set of reduced related areas realizes a graphic code whose non-zero coding elements or "spots" correspond to the reduced related areas. Each of these spots results from a random texturing of the photochromic material 2 within the subject material element in a related connected zone, which locally and uniquely modifies the structural characteristics of this connected zone, making its reproduction identical. impossible or almost impossible to scale.

The statistical characteristics of a spot are adjustable by adjusting the parameters of the generation of the light field 3 (parameters of the incident light beam 6, choice of the secondary material element 5 or of the zone which illuminates it, divergence of the light field 3 for example by playing on the parameters of an optics placed downstream of its generation) and / or on the position of the material element subject 1 in the light field 3. Subsequently, the statistical characterization in the signal acquisition of the random texture of a task gives the graphic code, after quantification, an additional dimension.

The re-registration of the same graphic code in the same place has structural characteristics at the level of the spots specific to the moment of photo-inscription which make it possible to distinguish the re-inscription of any previous inscription.

In order to guarantee the stability of the macroscopic state or the random texture of the photochromic material 2 obtained after exposure, the duration of insolation of said material is chosen sufficiently long to reduce almost all the photosensitive agents 2b. In practice, the exposure time is thus of the order of 1 to 3 minutes, depending on the photochromic material surface 2 to be insolated and the nature and concentration of photosensitive agents 2b distributed randomly within the matrix 2a of it. For cases where the insolation would not be long enough to reduce all of the agents 2b in the matrix 2a, the method of the invention advantageously proposes to eliminate them by chemical means and / or physico-chemical in order to achieve the macroscopic state stability of the desired photochromic material 2. Such a phase of elimination of the agents not reduced by insolation can in particular be done by treatment of the photochromic layer with an aqueous solution based on a thiosulfate.

According to another preferred characteristic of the process of the invention, it is also possible to prevent the unwanted photooxidation of the reduced agents 2b during the insolation by adding protective chemical entities such as thiol-type secondary agents. long carbon chain.

In the continuity of the random texturing method of a photochromic material described above, the invention also relates to and proposes in a second object a method for generating a new random arrangement of reduced photosensitive agents 2b after a first exposure in the matrix 2a. stable and porous photochromic material 2 of the subject material element 1 which has randomly modified the microscopic and macroscopic structures by a first light insolation.

The invention thus proposes in other words a method of erasing a first texture of the photochromic material 2 during a first insolation, then generating a new texture by a new insolation using a spatially variable light field.

According to the invention, this erasure can be achieved by insolation of the photochromic material element 1 by means of a light beam whose spectrum is located at least partially in the visible range. Once the previous random texture (resulting from the random arrangement of the photosensitive agents in the matrix) has been erased, it suffices to proceed to a new insolation of the photochromic material 2 of the subject material element 1 by a highly variable light field in the field. ultra-violet to inscribe a new figure in the photochromic material and thus provide a new texture and a new random arrangement of the photosensitive agents in the matrix of the photochromic material.

The invention therefore also proposes the possibility, thanks to this method of erasure and generation of a new random texture of the photochromic material, to modify unexpectedly and in a manner that is not completely controlled, the random macroscopic state of the material element. subject 1 without modifying the matrix 2a of the photochromic material 2 of this subject element 1.

After erasing an earlier random arrangement of the photosensitive agents 2b within the matrix 2a of the photochromic material 2, the generation of a new random macroscopic state of said material 2 from the same given figure or from a different figure depends on the subject material element 1, and more particularly its photochromic material 2 and, if appropriate, its previous macroscopic states. The use of a given figure either on a subject material element 1 given at two different insolation instants or on two different subject material elements leads to random arrangements of the photosensitive agents 2b in the matrix 2a of the different photochromic material 2. In addition, a change of intermediate material element 5 or more or less significant variation of the illumination conditions of this element by the light beam 6 of insolation also causes large variations in the random arrangement of the photosensitive agents 2b in the matrix 2a of the photochromic material 2 of the subject material element 1 after erasure of a first random texture in this material.

The two methods previously described in the context of the present invention find particularly advantageous outlets and applications in the field of the identification or authentication of data or goods, and more broadly the field of security based on the exploitation of data. properties of the material.

Therefore, a third object of the present invention relates to a method of securing and / or authenticating data (s) or object (s) which exploits the random macroscopic state conferred on a subject material element 1 comprising a photochromic material 2 according to one of the methods described above.

According to the security method of the invention, the random texture produced in the photochromic material 2 of a subject material element 1 is acquired with a suitable 3D, 2D or ID acquisition device, for example at least one CMOS sensor or CCD camera, and if necessary extract a signature, itself random, from the image of the random texture of the material element subject 1, and more precisely the surface of photochromic material previously acquired from it.

The random texture and / or the extracted signature thus constitute unfalsifiable identifiers of the subject material element 1 and, if necessary, can be used to secure data possibly associated with the subject material element. This texture and / or extracted signature have the peculiarity of representing the subject material element 1 given in a random macroscopic state fixed at a precise moment that is that of its insolation, possibly temporarily until transition to a new macroscopic random state acquired after erasure and new insolation.

In order to be able to exploit such a random texture for purposes of verifying objects or data secured by the method of the invention, it is appropriate as soon as a random texture is formed on a given subject material element to record, according to a secure protocol for example, the random texture and / or the extracted signature. This record will serve as a reference, either in a secure database or on the subject material itself by registration after encryption, of the reference. Subsequently, it will thus be possible to check, by appropriate reading means, whether the texture and / or the signature extracted at a verification time T corresponds to the reference originally recorded, and thus validate or not the identity of the object or decrypt data, verify their integrity, their origin by a random texture in accordance with the methods of the invention. According to an advantageous characteristic of the security method of the invention, the random macroscopic state of the subject material element 1 can be self-reconfigurable by extracting from this random macroscopic state a non-predictable signature within the meaning of patent FR 2 866 139, and according to methods described in this patent, and using it after normalization via a look-up table to choose a set of illumination parameters of the secondary hardware element 5 to generate a new random texture of the element's photochromic material. subject material.

In order to make more concrete the feasibility of the methods of the invention, a concrete example of implementation of these methods is presented below.

The invention is based on the structural modification of a photochromic material by light insolation, it is first necessary to choose this material, which will then be affixed to a material element subject 1 to identify or secure. Preferably, in the context of the invention, the photochromic material 2 used to support random macroscopic state variations and to allow their transition is a thin layer of titanium dioxide TiO ₂ mesoporous, constituting the matrix 2a of the material 2, and containing silver ions Ag ⁺ , which constitute the photosensitive agents 2b. This layer of mesoporous TiO ₂ containing Ag ⁺ ions is preferably prepared by sol-gel route. The layer obtained is homogeneous and colorless at the observation scale (greater than one hundred nanometers in optical microscopy).

The microscopic state of the layer 2 obtained is shown diagrammatically in FIG. 4. It distinguishes the solid matrix 2a from porous TiO ₂ , whose pores have a nanometric size and a very narrow size distribution, and containing the photosensitive agents 2b. Ag ⁺ silver, the latter being randomly distributed in the matrix 2a uncontrollably and unpredictably. The size of the pores and agents is small enough that they are invisible at the macroscopic scale. This type of material 2 has well-known photochromic properties allowing a reversible local coloring, under light beam 6, of the material in its mass.

The initial material 2, colorless, takes a yellowish coloration in reflection on the observation scale after illumination by a UV light radiation. FIG. 7 illustrates the random color variations obtained after insolation of the sample of the material 2 obtained previously for 30 seconds with a laser 6 emitting at the wavelength of 244 nm a power of 60 mW, scattered by a rough surface . This coloration is due to the plasmon resonance of the silver nanoparticles formed in the layer by photo-reduction of the ionic precursors.

At the microscopic scale, as represented in FIG. 5, the silver nanoparticles formed under LJV light excitation preferentially grow in the pores of the matrix 2a, which tend to limit their size. The small size dispersion of the nanoparticles results in a yellow coloration characteristic of an absorption spectrum of limited width in the visible range.

The generation of a macroscopic random state in the material is effected by insolating the sample by means of a scab figure, itself obtained by illuminating a diffusing intermediate material element, such as a frosted silica glass, by a laser beam 6 emitting at the wavelength 244 nm. The scab image generated by the illumination of the scattering element 5 is impossible or almost impossible to reproduce without this scattering element, which is itself impossible or almost impossible to reproduce physically.

The marking device shown in FIG. 6 comprises the laser source S, the diffusing element 5 and the photochromic material 2 on the subject material element 1 to be identified or secured, for example. It may also include focusing and imaging optics placed on either side of the diffusing element 5. The resolution of the scab pattern is modifiable, for example by changing the distance between the scattering element 5 and the 'element material subject 1. The resolution can thus be adapted to the observation scale. Figures 8A and 8B show two figures of scab inscribed with different characteristic dimensions and observed at the same scale. The insolation of the material by a laser light modulated by the scattering element 5, thus randomly on the observation scale, induces firstly the migration of the Ag ⁺ ions towards the zones receiving the most luminous intensity and on the other hand the growth of reduced silver nanoparticles in these areas. These phenomena are reflected in the observation scale by the formation of yellowish stains (lighter spots in the figures), the shape and extent of which depend both on the projected scab pattern and on the ion concentration. Ag ⁺ present locally in areas of higher intensity. The unstained areas are characteristic of the absence of reduced money and are also relatively empty of precursors.

At the end of the first insolation of the material, the spatial distribution of the silver nanoparticles and the possibly unmodified silver ions in the matrix 2a of the layer 2 thus becomes inhomogeneous on the observation scale. Furthermore, as proposed by the random texturing method of the invention, stabilization against unwanted reduction of unreduced Ag ⁺ silver ions can be achieved by rinsing in an aqueous solution containing a thiosulfate. Similarly, stabilization against unwanted oxidation of the reduced silver may be made by a long chain carbon thiol type agent.

In accordance with the method of generating a new random texture of the invention, the photochromic material 2 resumes its initial colorless (transmission) appearance after exposure by a homogeneous visible light beam. This insolation is effected by means of a laser beam, of wavelength belonging to the absorption band of the material after UV insolation, or typically around 488 nm or 514 nm, and having a power density typically of the order of W.mm- ² . The exposure time necessary for the complete erasure of the random texture, where appropriate of the prior written code is typically of the order of one minute and depends on the conditions used when writing the scabbing figure inscribed by the first insolation in UV light. Figures 9A and 9B show two images taken at the same place before and after insolation at 488 nm. The random spot distribution of the 9A image was obtained by irradiating the photosensitive layer for 2 min at the wavelength of 244 nm using a laser output power of 100 mW and a scattering sample placed at 3.7 cm of the photosensitive material. This distribution of spots is totally erased (FIG. 9B) after homogeneous irradiation of the zone at the wavelength of 488 nm for 2 minutes using a power of 1 W / mm ² .

The disappearance of the color is due to the oxidation (and dissolution) of the silver nanoparticles formed during the inscription stage. This oxidation (and dissolution) is made possible by the photocatalytic effect of TiC ^ and by the presence of oxygen in the matrix with which the silver nanoparticles formed during insolation under light at 244 nm wavelength exchange electrons. This oxidation (and dissolution) does not completely call into question the inhomogeneous nature of the distribution of silver in the layer induced by the previous UV exposure.

The formation cycle (generation of the macroscopic random state), oxidation / dissolution of the nanoparticles (erasure of the macroscopic random state) can be repeated in order to achieve a random macroscopic state transition. A series of five cycles was performed demonstrating the possible writing of five random distributions of different spots at the same location on a sample as shown in Figure 12.

In the event of prior stabilization with respect to the unwanted reduction of unreduced silver, it is necessary to reload the TiO ₂ matrix of the photochromic material with silver salt. In case of prior stabilization against the undesired oxidation of the reduced silver, the long-chain carbon thiol-type agent can be decomposed under the effect of UV insolation in the presence of the matrix of TiCb. The generation of a random macroscopic state from a given scab pattern depends on the subject material element, and more particularly on its photochromic part, and possibly on previous macroscopic state transitions. The use of a given scab figure, either on a subject matter element given at two different generation times, or on two different material subject elements, leads to different and similar scabbed scabs. The change of secondary element or the more or less significant variation of the illumination conditions of the secondary element causes a random macroscopic state transition such that macroscopic states before and after transition are strongly different and independent.

It is also possible to spatially limit the photo-inscription of a scab figure by means of a mask consisting for example of a diaphragm of size (and possibly shape) adjustable (s), placed in the light field 3 of insolation upstream of the material element subject 1.

The random texturing of a given spot of a graphic code inscribed in several related areas of the photochromic material 2 on a subject material element 1 can be achieved by translating the diaphragm into a field of scab after, transversely, the desired lateral position for the stain and, in depth, typically a few millimeters / centimeters, the desired average size for scab of the stain. For an optical microscope observation and a matrix camera acquisition with a resolution of 0.25 μm / pixel, the photo-inscription of about ten micrometers allows the reading of a "datamatrix" type graphic code with a quantization of the average size of apple scab on at least ten levels. Thus the texturing of such a graphic code, two-dimensional at the origin, makes the three-dimensional code.

Claims

1 - Method of random texturing of a material element (1) comprising a photochromic material (2) comprising a stable matrix (2a) in which are distributed in a chaotic manner (2b) photosensitive agents, characterized in that the the photochromic material (2) is insulated by means of a random variable field (3) and random causing the reduction of the photosensitive agents (2b) within the matrix (2a) and their random migration to reach a new random arrangement of said agents photosensitive in said matrix. 2 - Process according to claim 1, characterized in that the illuminating light field (3) of the photochromic material element (1) results from the light transmitted and / or reflected by an intermediate material element (5) placed on the optical path of an incident light beam (6) whose physical characteristics spatially vary the intensity of the light field (3).

3 - Process according to claim 1 or claim 2, characterized in that the variable and random light field can also result from the light transmitted and / or reflected by an optical system and / or digital using a hardware or digital key. 4 - Process according to claim 2, characterized in that the incident light beam (6) is a coherent or quasi-coherent light beam, the resulting light field (3) being a scabbed field.

5 - Process according to claim 2, characterized in that the incident light beam (6) is a structured incoherent light beam.

6 - Process according to one of claims 1 to 5, characterized in that one varies the intensity spatially and / or the divergence of the insolation light field (3) through an optical system.

7 - Method according to one of claims 1 to 6, characterized in that the insolation of the photochromic material element (1) is performed by means of a light field (3) whose spectrum is at least partially located in the ultra-violet field. 8 - Method according to one of claims 1 to 7, characterized in that is stabilized, where appropriate, the random arrangement of photosensitive agents (2b) in the photochromic material element after insolation on the one hand by a chemical and / or physical treatment intended to eliminate the non-reduced photosensitive agents and / or on the other hand by a physical and / or chemical treatment intended to protect the reduced photosensitive agents from parasitic light rays.

9 - Process according to one of claims 1 to 8, characterized in that the photochromic material element (1) comprises a matrix (2a) of titanium oxide (TiO2) porous with a pore size of less than 10 ² nanometers and containing photosensitive agents (2b) such as silver salts, gold or platinum, or even silver complexes.

10 - Process for generating a new random arrangement of reduced photosensitive agents (2b) in a stable matrix (2a) of a photochromic material element (1) whose structure has been modified according to the random texturing method of claims 1 at 9, characterized in that the random arrangement of reduced photosensitive agents obtained after a first insolation of the photochromic material element is erased and in that a further insolation is carried out using a spatially variable light field.

11 - A method of generating a new random arrangement according to claim 10, characterized in that the prior random arrangement of the photosensitive agents reduced by insolation of the photochromic material element by means of a light beam whose spectrum is located at least partially in the visible range.

12 - A method of generating a new random arrangement according to one of claims 10 or 11, characterized in that, before erasure, if necessary is removed the protection of reduced photosensitive agents introduced before the stabilization of said agents performed according to the method of claim 8 and / or the matrix of the photochromic material element is reloaded into unresolved light-sensitive agents. especially following an elimination of such agents during stabilization according to claim 8.

13 - Method for securing and / or authenticating data (s) or object (s) in which: a) a figure of a spatially variable light field is written in a photochromic material element according to the method; random texturing method defined in one of claims 1 to 9, said figure being deformed according to the arrangement of the photosensitive agents and imparting a random texture to the photochromic material element; b) the photochromic material element is probed; c) extracting a signal or an image of the random texture and / or a signature of this texture to secure an object and / or data.

14 - security and / or authentication method according to claim 13, wherein in step c) the signal or the image of the random texture and / or the extracted signature is recorded as a reference in a database and / or on the photochromic material element or on an object to be secured and / or authenticated, then:

the photochromic material element is again probed;

15 - Securing method and / or authentication according to claim 14 wherein the step of registration and / or authentication is done according to a secure protocol.

16 - Method for securing and / or authentication of data or object (s) according to claim 13 or 14 wherein the photochromic material is reconfigured by erasing the prior random texturing of the photochromic material element according to the method of claims 10 at 12 and then proceed to steps a) to c). 17 - security and / or authentication method according to one of claims 13 to 16, wherein a random signature is extracted from the random arrangement of reduced photosensitive agents in the photochromic material element, the random signature constituting a key or a cryptographic function parameter (s) and / or steganographic (s).

18 - security and / or authentication method according to one of claims 13 to 17, wherein the photochromic material is deposited on at least part of an object to secure and / or authenticate.

19 - Graphic code composed of an arrangement of unitary elements determined, characterized in that at least one unitary element is textured according to the method of claims 1 to 9.